A tape automated bonding (TAB) package consists of bonding a large chip bonded to metal foils. The conductive pads at the edge of the package are attached by soldering, welding or conductive epoxying to the printed circuit boards or substrates. The bonding of the pads (leads) of the metal foil to the chip is known as inner lead bonding (ILB) and the bonding of the pads (leads) to the printed circuit boards or substrate matching pads is known as outer lead bonding (OLB).
Though tape automated bonding has been in existence for about two decades, the prime thrust as yet has been on low lead count (less than 100 leads, typically 16 to 32 leads) TAB devices. The recent technological development in VLSI chips with denser Input/Output and surface mounting of packages with finer lead pitch has generated tremendous interest in high lead count (200+leads) TAB packages. As the number of leads on TAB devices is increasing, the pitch (distance between the center lines of two adjoining leads) is getting smaller. This poses new challenges to the manufacturing, testing, reliability, performance, etc. of such packages.
The present invention is directed to an apparatus and a method for inspecting outer lead bond joints for lifted leads and for repairing them in situ. It is within the scope of this invention to extend the technique to inner lead bonds and similar tasks.
After TAB devices are aligned and bonded to the corresponding pads of the substrates, many times cold solder joints or solder failures occur. This is due to the fact that the bonding head does not apply enough pressure or even pressure on all leads and/or due to the fact that enough heat is not transferred to the leads and to the solder to form a good bond. Such conditions are referred to as lifted leads. In the case of a cold solder joint, a narrow break or fissure appears either between the lead and solder or between the solder and pad due to lack of solder reflow. A solder failure means there is no bond between the lead and the pad which, again, is due to the lack of solder reflow. Typically, this will, create air gaps in between the lead and pad but, in some cases, it can also end up with no air gaps.
Existing visual techniques for either inspecting or repairing lifted leads of TAB devices are generally unsatisfactory. The known inspection systems have failed to inspect lifted leads because of their inability to either identify the lifted leads or to differentiate between a good and a lifted lead.
Generally, in high lead count TAB devices the width of the lead-pad may be in the order of a few to several mils. The effective length of the outer lead bond may be in the order of 20 to 80 mils. When the joints (lead-solder-pad combination) are viewed under high power magnification, from the top, generally with a collimated light source, the pad on the PCB (substrate) and the solder underneath are hidden by the TAB lead on the top. Even when viewed from an angle, the dispersion of the light from the surfaces is such that it is difficult to distinguish among the surfaces of the lead, the solder and the pad. Further, where a lead is not soldered properly, the pad may contain solder and solder may also be on the sides or under surface of the lifted lead. Current viewing systems have difficulty distinguishing among these surfaces.
Broadly, the invention comprises a plurality of light sources adapted to illuminate at least one TAB joint which comprises a first light source of collimated light disposed above and in front of the sides of the surfaces of the joint under investigation; a second source of fluorescent light disposed above and behind the joint; and a third source of fiber optic light, said fiber optic light comprising at least two bundles whereby two beams of fiber optic light illuminate the sides from a third angle(s). The angular relationship of the light sources illuminating the respective sides of the surfaces of the lead-solder-pad is such that surfaces may be readily distinguished.